1. Field of the Invention
The subject invention relates to a method of mounting a light emitting diode to a substrate to be connected to a circuit to be electrically driven.
2. Description of the Prior Art
The electrically driven light emitting diode (LED) assemblies of the prior art typically include an electrically and thermally conductive heat dissipater or heat sink sandwiched with an electrically insulating circuit board. A plurality of spaced lengths of circuit traces is disposed over the insulative circuit board to prevent electrical conduction between the traces and the heat sink. A plurality of light emitting diodes is mounted on the circuit board and have electrical leads for electrical contact with the traces for powering the LEDs.
An example of such a prior art assembly is disclosed in U.S. Pat. No. 5,857,767 in the name of the inventor Peter A. Hochstein named herein. In that assembly, an insulating coating is disposed over an aluminum heat sink with circuit traces on the insulating layer and the leads of the LEDs are adhesively secured to the circuit traces to hold the LEDs in position. In other U.S. Pat. Nos. 6,045,240; 5,785,422 and 5,782,555, all in the name of Peter A. Hochstein, the lateral extending leads of the LEDs are secured to the circuit traces by soldering or an adhesive. Other methods for securing lamps to a circuit board include using metallic terminals that are spring biased into a retaining position with the circuit board, as disclosed in U.S. Pat. No. 5,513,082 to Asano.
Current practice is also exemplified by the Hewlett-Packard xe2x80x98Barracudaxe2x80x99 (HPWLB x 01) LEDs and the HPWL-MDXX family of LED arrays. As shown in the pertinent H. P. data sheets for this device, eighteen individual xe2x80x98Barracudaxe2x80x99 LEDs are mounted onto a metal core circuit board by adhesive means. The prior art methods are costly and difficult to implement in production, as each LED attachment must be done separately. Furthermore, the adhesive attachment of the heat sinked LEDs to a circuit board, rather than to the metallic heat dissipater, is far from optimal. This is because virtually all the advanced high performance LEDs incorporate some form of low thermal resistance path for heat rejection from the LED die. The junction temperature of the die depends not only on the thermal impedance of the LED lead frame or the integral heat sink, but also on the thermal resistance of the LED heat sinkxe2x80x94heat dissipater interface and the thermal resistance of the dissipater itself.
Electronic components that generate heat are often attached to heat dissipaters using rivets, screws and spring clips. Examples of clips are disclosed in U.S. Pat. No. 4,959,761 to Critell et al, U.S. Pat. No. 5,186,535 to Yokoyama, U.S. Pat. No. 5,264,998 to Bax and U.S. Pat. No. 5,440,468 to Savage, Jr. In order to enhance thermal conduction at the interface of the component and the heat dissipater, materials are often sandwiched between the component and the heat dissipater. A variety of filled or unfilled greases, gels or viscoelastic materials can reduce the thermal impedance of the interface (joint) in order to maximize heat transfer. Microscopic gaps (typically air) between the hot component and the heat dissipater interfere with the efficient transfer of heat to the sink. Materials with thermal conductivities significantly higher than air fill the voids and can markedly improve the thermal performance of the interface. However, it has been shown that optimal heat transfer is not achieved unless a given contact force is maintained between the hot component and the heat dissipater.
In conventional electronic practice where heated components are often clamped to heat sinks with screws and insulating washers, it is common to note a decrease in heat transfer efficacy over time. This loss of heat transfer is particularly apparent if the devices are operated at higher temperature (in excess of 85xc2x0 C.) and if plastic insulators are used in the fastener chain. Long term creep or plastic deformation of the attachment means may reduce the original clamping force to a fraction of the original value, thereby compromising heat flow across the mounting interface, which may lead to thermally induced failure.
Adhesive attachment systems are generally more resistant to high temperature operation, but sometimes exhibit detachment due to repeated temperature cycling. The relatively large differences in thermal expansion coefficients of plastics, epoxies and metals can give rise to substantial stresses at the attachment region. These stresses may exceed the strength of the adhesive and may eventually result in joint failure.
The advent of very high performance light emitting diodes (LEDs) with integral heat sinks require special mounting of these devices in order to achieve the desired heat rejection from the LED. In order to utilize the inherent low thermal resistance that is available in this newer generation of high performance LEDs, special attention must be given to the attachment of these LEDs to the heat dissipater. Such attachment must obviously include means to secure a very low thermal impedance interface between the LED heat sink and the heat dissipater to which it is connected.
The subject invention, therefore, provides a method of assembling an electrically driven light emitting diode (LED) having electrical leads in electrical contact with circuit traces that are disposed in predetermined spaced lengths over an electrically insulating layer. The method is characterized by mechanically holding the LED in a predetermined position on the insulating layer with the electrical leads thereof in electrical engagement with the traces.
The resulting electrically driven light emitting diode (LED) assembly is, therefore, characterized by an independent holding device mechanically holding each of the LEDs in a predetermined position on the insulating layer with the electrical leads thereof in electrical engagement with the traces.
The present invention solves the problems associated with the prior art retention systems by employing an independent holding device for mounting each LED in position to most effectively connect the leads thereof to the circuit and to transfer heat to the heat dissipater.